ubuntu更新源的替换方法

deb http://mirrors.ustc.edu.cn/ubuntu/precise main restricted universe multiverse

deb http://mirrors.ustc.edu.cn/ubuntu/precise-security main restricted universe multiverse

deb http://mirrors.ustc.edu.cn/ubuntu/precise-updates main restricted universe multiverse

deb http://mirrors.ustc.edu.cn/ubuntu/precise-proposed main restricted universe multiverse

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